I bought this lens to replace my Canon 17-85 IS, which I found lacked sharpness. I was not disappointed. This Tamron lens can take immensely sharp and detailed images, and to date I have not found a sharper more detailed lens.
I also own the Canon 50mm f/1.8 II and 70-200 f4 IS. Some claim the 70-200 is the sharpest zoom lens ever produced. While this lens produces more "keepers" than my Tamron, has faster more accurate focus, and better ergonomics, the Tamron can produce sharper, more detailed photos.
This has been especially true since I upgraded to the Canon 7D (18mp) from the 30D (8mp). In it's sweet spot, the Tamron can keep up with the 18mp resolution - the Canon lens cannot. I'd say the charts at SLRgear.com pretty much exactly match my experience in this regard - and they back up what I say here.
The trick with the Tamron is to focus manually in low contrast scenes. However with a bright contrasty target, the Tarmon AF is deadly accurate.
The Tamron is always on my camera. I only change to other lenses in special situations. It is light, compact and well built. I don't treat my lenses particularly well, and the IQ on the Tamron has not suffered from its many bumps and bruises.
My father has also bought one in pentax mount to replace a whole bag full of lenses and has been similarly impressed.
For amateurs looking to catapult their photography to the next level, or for pros who don't rely on an "L" badge to find customers - I'd highly recommend this lens.